Even when I exited and restarted BOINC the first two in the list were being used rather than the 2 GTX 460s. The above order is the order in which cuDeviceGet() returns them so my guess is that BOINC should be changed to sort based on the GFLOPS peak and use the fastest ones before the slower ones.
I know the two slow ones don't do much but my having them it effectively means I get 800 WUs for each GTX 460 to munch rather than 400 so can survive longer when the servers are away.